cmd/go/internal/load: join incompatible and dirty build specifiers with .

Change "+incompatible+dirty" version to be "+incompatible.dirty" such
that it is SemVer spec compatible.

Fixes #71971

Change-Id: I714ffb3f1ad88c793656c3652367db34739a2144
Reviewed-on: https://go-review.googlesource.com/c/go/+/652955
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Michael Matloob <matloob@golang.org>

src/cmd/go/internal/load/pkg.go

@@ -2585,7 +2585,12 @@ func (p *Package) setBuildInfo(ctx context.Context, autoVCS bool) {
 		vers := revInfo.Version
 		if vers != "" {
 			if st.Uncommitted {
-				vers += "+dirty"
+				// SemVer build metadata is dot-separated https://semver.org/#spec-item-10
+				if strings.HasSuffix(vers, "+incompatible") {
+					vers += ".dirty"
+				} else {
+					vers += "+dirty"
+				}
 			}
 			info.Main.Version = vers
 		}

src/cmd/go/testdata/script/build_version_stamping_git.txt

@@ -108,6 +108,19 @@ go version -m example$GOEXE
 stdout '\s+mod\s+example\s+v1.0.3-0.20220719150703-2e239bf29c13\s+'
 rm example$GOEXE
 
+# Create +incompatible module
+exec git checkout v1.0.4
+exec git rm go.mod
+exec git commit -m 'commit 6'
+exec git tag v2.0.0
+exec git checkout HEAD^ go.mod
+# And make the tree +dirty
+mv README4 README5
+go build
+go version -m example$GOEXE
+stdout '\s+mod\s+example\s+v2.0.0\+incompatible.dirty\s+'
+rm example$GOEXE
+
 -- $WORK/repo/go.mod --
 module example